Along with the size reduction of electronic devices, electronic parts mounted on a printed wiring board are becoming smaller in size and thickness. For instance, there is a tendency that a power converting transformer formed by winding a coil around a bobbin is converted to a thin sheet-like transformer.
The sheet-like transformer includes, e.g., a coil pattern formed on a printed wiring board, a pair of transformer cores arranged to grip the coil pattern therebetween and a clip or a fixing spring for joining the transformer cores together (see, e.g., Japanese Patent Application Publication No. 2005-175064). The transformer cores include, e.g., an I-core and an E-core. An inductance value is controlled by adjusting a gap length of outer leg portions or a central leg portion of the E-core.
In the sheet-like transformer disclosed in the above-cited reference, however, the I-core and the E-core are configured to have the same external dimension. For this reason, the I-core and the E-core are likely to get out of alignment with each other. As a result, the amount of leaked magnetic fluxes is increased in the transformer, which may possibly lead to a transformation loss. In an extraordinarily high temperature region, there is a likelihood that a circuit may be destroyed due to an increase in transformation loss and an increase in electric current caused by magnetic saturation.